Surround sound or audio reproduction from various positions about a listener can be provided using several different methodologies. One technique uses multiple speakers encircling the listener to play audio from different directions. An example of this is Dolby® Surround Sound, which uses multiple speakers to surround the listener. The Dolby 5.1 process digitally encodes five channels (plus a subwoofer) of information onto a digital bitstream. These are the Left Front, Center Front, Right Front, Surround Left, and a Surround Right. Additionally, a Subwoofer output is included (which is designated by the “0.1”). A stereo amplifier with Dolby processing receives the encoded audio information and decodes the signal to derive the 5 separate channels. The separate channels are then used to drive five separate speakers (plus a subwoofer) placed around the listening position.
Dolby 6.1 and Dolby 7.1 are extensions of Dolby 5.1. Dolby 6.1 includes a Surround Back Center channel. Dolby 7.1 adds left and right back speakers that are preferably placed behind the listening position and the surround speakers are set to the sides of the listening position. An example of this is provided in FIG. 1. Referring now to FIG. 1, the conventional Dolby 7.1 system includes Left Front (LF), Center, Right Front (RF), Left Surround (LS), Right Surround (RS) Back Surround Left (BSL) and Back Surround Right (BSR). Additionally, a Subwoofer, or Low Frequency effects (LFE), is shown.
Upon playback, decoders at the audio amplifier decode the encoded information in the audio stream and break up the signal into its constituent channels—e.g., 7 channels plus a subwoofer output for Dolby 7.1. The separate channels are amplified and sent to their respective speakers. One downside of Dolby 7.1 and other multi-speaker surround sound systems is that they require more than two speakers. Moreover, such multi-speaker surround sound systems require placement of the speakers around the listening environment. These requirements can lead to increased cost, additional wiring and practical difficulties with speaker placement.
Additionally, the sound created by the conventional speakers is always produced on the face of the speaker (i.e., at the speaker cone). The sound wave created at the surface propagates through the air in the direction at which the speaker is pointed. In simplest terms, the sound will appear to be closer or farther away from the listener depending on how far away from the listener the speaker is positioned. The closer the listener is to the speaker, the closer the sound will appear. The sound can be made to appear closer by increasing the volume, but this effect is limited.
In a surround sound speaker system using conventional speakers, speakers may be placed to ‘surround’ the listener, but it is apparent that the sound is produced at discrete points along the perimeter corresponding to the position of the speakers. This is apparent when listening to content in a surround-sound environment. In such environments, the sound can appear to move from one speaker to another, but it always sounds like its source is the speaker itself—which it is. Phasing can have the effect of blending sound between speakers, but conventional surround sound systems cannot achieve placement or apparent placement of sound in the environment at determined distances from a listener or listening location.
Moreover, even this limited ‘surround’ effect cannot be achieved with only a pair conventional speakers. Introducing audio processing effects to a two-channel (Left/Right) system can allow the sound to appear to move from the left speaker to the right speaker, but the sound cannot be placed at a desired distance from or beyond the listener.